LEUCKART, ON MICROPYLE. 291 



some experience in the detection of this apparatus, he has, 

 nevertheless, in many cases been compelled to make pro- 

 longed and often-repeated examination, and to devote the 

 closest attention to the subject, before satisfying himself with 

 respect to its existence and conformation. Had he not 

 entertained, a priori, the firmest conviction of its existence, 

 it would in many Insects certainly have escaped him. 



On the other hand, however, he says we must not expect 

 that the existence of a chorion is in all cases associated with 

 a micropyle. The chorion may possibly not be formed until 

 after the fertilizing contact with the spermatic filaments has 

 taken place, as in the Turbellariae, Trematoda, and probably 

 also in the Cestoidea, in which, to judge merely from the 

 anatomical conformation of the sexual organs, the spermatic 

 Jilaments are enclosed at the same time with the vitelhis and 

 the germinal vesicle, in a hard chorion-like capsule. Ac- 

 cording to Mcissner, the ovum of Gammariis has a micropyle 

 only in the vitelline membrane, over which the chorion is 

 continued ; in this case, impregnation, without doubt, takes 

 place also before the deposition of the chorion. 



We have tlms sought to refer the physiological necessity of 

 a micropyle at once to the physical condition of the egg- 

 membranes which are formed before impregnation has taken 

 place. But, at the same time, it can by no means be said 

 that such a provision is exclusively confined to ova furnished 

 with firm membranes. There remain numerous other con- 

 ditions which, even in the case of a soft and delicate covering 

 to the ovum (Meissner expressly remarks of the vitelline 

 membrane in Gam?narus, which has a micropyle, that it is 

 " excessively delicate "), render the existence of a micropyle, 

 if not absolutely necessary, still advantageous. But in any 

 case, the occurrence of the micropyle under these circum- 

 stances will, of course, be far more limited than in ova having 

 a hard and less penetrable covering. 



Thus there may be said to be three distinct modes in 

 which impregnation, that is to say, the contact of the 

 spermatic filaments with the vitellus, is brought about : — 



1. The entrance of the spermatic filaments, with 

 penetration of the egg-covering ; 



2. Penetration through micropyles ; and 



3. Penetration into the vitelline mass before the 

 deposition of the membranes of the ovum. 



To which may be added a fourth mode of contact, lately 

 pointed out, more particularly by Meissner : — 



